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V 


MODEL  OF  AN 
ARIZONA  GOLD  MINE 


'■^^-..Js^ 


FIELD  MUSEUM  OF  NATURAL  HISTORY 

CHICAGO 
1922 


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Field  Museum  of  Natural  History 

DEPARTMENT  OF  GEOLOGY 
Chicago.  1922 

Leaflet  Number  1 

Model  of  an  Arizona  Gold  Mine 

This  model  represents  a  small  gold  mine  of  med- 
ium richness  in  a  vertical  quartz  vein  six  feet  wide. 
The  vein  and  mine  workings  are  presented  in  section, 
that  is,  as  they  would  appear  if  they  were  cut  through 
along  the  center  of  the  vein  and  one-half  removed. 

This  model  is  intended  to  illustrate  the  ordinary 
features  present  in  some  form  in  the  great  majority  of 
all  metal  mines  and  to  show  the  orderly  manner  in 
which  mines  are  developed  and  the  ores  extracted. 

Great  diversity  in  the  nature  and  structure  of  ore 
deposits  compels  a  corresponding  variety  in  the  details 
of  workings  by  means  of  which  the  ores  of  these  de- 
posits are  extracted. 

However  diverse  these  details  may  be,  they  all 
may  be  grouped  as  parts  of  major  features  which  are 
common  to  nearly  all  metal  mines. 

In  this  respect  these  excavations  resemble  the 
more  familiar  structures  on  the  surface.  As  build- 
ings, however  much  as  they  may  differ  in  appearance, 
are  all  built  up  of  units,  as  walls,  floors,  windows  and 
doors,  so  these  underground  workings  are  composed 
of  groups  of  such  units,  as  drifts,  cross-cuts,  stopes, 
raises,  etc. 

There  are  some  exceptions  to  this  rule.  The  great 
open-cut  mines  of  the  Great  Lakes  Iron  Ranges,  for 
instance,  are  merely  great  quarries.  In  certain  de- 
velopments of  caving  systems  of  mining  some  features 
of  the  ordinary  type  are  nearly  unrecognizable  or 
quite  absent.    Coal  mines  and  mines  worked  along  the 

[1] 


K«tiirsl  mtt»nr  WSew3 


2  FiKM)  Museum  of  Natural  History 

lines  of  coal  mines  have  developed  a  nomenclature  of 
their  own  for  many  of  these  features. 

The  mine  represented  in  this  model  is  a  small  one 
in  a  vertical  vein  in  hard  rock.  The  rock  is  very  firm 
at  the  left  near  the  shaft  and  becomes  less  solid  to  the 
right,  where  the  vein  is  intersected  by  another  one  in  a 
weaker  and  more  broken  rock  which  tends  to  cave  and 
fill  any  openings  made  in  it.  The  mine  is  in  a  region 
where  timber  is  expensive  and  where  there  is  but 
little  water.  Electric  current  developed  by  water 
power  is  available  for  power.  The  methods  of  mining 
employed  are  those  suited  to  these  conditions. 

The  vein  forms  the  entire  vertical  front  of  the 
model.  Its  intersection  with  the  surface  of  the  ground 
is  its  OUTCROP.  Near  the  shaft  and  for  the  greater 
part  of  its  length,  the  outcrop  is  covered  by  soil. 
Towards  the  right  of  the  model  the  outcrop  of  the 
vein  appears  above  the  soil  in  two  places,  where  it  can 
be  distinguished  from  the  surrounding  rock  by  its 
darker  color.  This  dark  brownish-red  color  is  due  to 
the  presence  of  large  quantities  of  oxides  of  iron  which 
are  residues  left  from  the  destruction  by  weathering 
of  some  of  the  original  vein  minerals.  Such  an  iron- 
stained  outcrop  is  called  an  IRON  HAT. 

Not  all  veins  develop  an  iron  hat,  but  most  of 
them  present  at  their  outcrops  color  contrast  with  their 
surroundings  by  which  they  may  be  readily  traced 
wherever  they  are  not  covered  by  soil. 

THE  ORE  in  the  upper  part  of  the  mine  is  a 
weathered  and  iron-stained  quartz.  The  values  lie  in 
minute  particles  of  free  gold  disseminated  through 
this  quartz.  In  the  lower  levels  this  ore  is  replaced, 
as  is  usual  in  such  deposits,  by  a  cleaner  quartz,  which 
contains  no  free  gold,  but  encloses  quantities  of  the 
sulphides  of  the  heavy  metals,  principally  iron  and 
copper.     The  two  principal  sulphides  in  this  vein  are 

[2] 


Model  of  an  Arizona  Gold  Mine  3 

the  bright,  yellow,  glittering  pyrite  or  iron  pyrite 
composed  of  iron  and  sulphur  and  the  similar  but 
deeper  yellow  chalcopyrite,  composed  of  iron,  copper 
and  sulphur. 

The  gold  in  this  part  of  the  mine  is  thinly  dissem- 
inated in  invisible  form  through  the  sulphides.  The 
quantity  of  gold  is  very  small.  On  the  average,  two 
ounces  of  gold  will  be  scattered  through  a  ton  of  the 
sulphides.  Any  substance  of  ordinary  value  so  thinly 
distributed  would  be  worthless,  as  the  expense  of 
blasting  and  raising  the  ore  would  much  exceed  what 
it  could  be  sold  for.  The  value  of  gold  is  so  great, 
however,  that  even  the  small  quantity  in  a  ton  of 
sulphides  is  worth  forty  dollars,  a  value  that  well 
repays  working. 

THE  GANGUE  is  the  barren  quartz  which  en- 
closes the  ore.  In  practice  the  terms  ore  and  gangue 
are  somewhat  loosely  used.  Thus  one  may  describe 
this  material  as  sulphide  ore  in  quartz  gangue  and 
another  may  describe  it  as  a  quartz  and  sulphide  ore. 
Also  the  sulphide  mineral  with  such  quartz  as  is 
extracted  with  it  in  the  ordinary  process  of  mining  is 
commonly  referred  to  as  the  ore  and  only  that  quartz 
which  is  extracted  separately  or  is  free  from  the  sul- 
phides is  called  the  gangue.  The  gold  is  not  equally 
distributed  through  the  sulphides.  In  some  places  the 
sulphide  will  contain  much  more  than  the  average 
quantity  of  gold  and  in  other  places  it  will  carry  little 
and  in  some  places  none.  The  sulphides  without  value 
are  not  spoken  of  as  gangue.  They  are  called  poor  or 
barren  ore.  In  the  upper  levels,  where  the  sulphides 
are  absent,  the  ore  is  that  part  of  the  quartz  which 
contains  free  gold,  while  the  valueless  portions  of  the 
quartz  form  the  gangue. 

One  of  the  objects  of  a  good  miner  is  to  extract 
as  much  of  the  good  ore  as  possible  and  at  the  same 

[3] 


4  FiF.i.D  Museum  of  Natural  History 

time  to  leave  as  much  as  possible  of  the  worthless 
material  behind,  for  it  costs  as  much  to  mine  and 
raise  gangue  as  it  does  to  mine  and  raise  ore. 

If  the  sulphides  are  worth  forty  dollars  a  ton 
and  it  is  necessary  to  mine  three  tons  of  quartz  with 
each  ton  of  ore,  then  the  material  mined  is  worth  only 
ten  dollars  a  ton,  a  value  that  in  so  small  a  mine  in 
so  remote  a  region  will  barely  pay  expenses  of  mining 
and  treatment,  although  larger  mines  find  such  ore 
very  profitable. 

When,  as  is  the  case  here,  the  ore  can  be  so  mined 
that  with  each  ton  of  sulphide  only  one  ton  of  quartz 
must  be  mined,  the  material  hoisted  is  worth  twenty 
dollars  a  ton  and  yields  a  good  profit. 

As  the  ore  is  not  distributed  equally  in  all  parts 
of  the  vein,  there  must  be  certain  areas  where  the  ore 
is  richer  than  elsewhere.  These  areas  are  likely  to 
have  some  regularity  of  form  and  distribution.  The 
laws  controlling  their  distribution  are  frequently  very 
obscure,  so  that  in  many  mines  the  discovery  of  these 
enriched  areas  before  the  workings  actually  cut  them 
is  very  difficult  and  often  impossible.  They  frequently 
assume  very  elongated  forms,  and  are  then  called 
CHUTES  or  SHOOTS  or  sometimes  CHIMNEYS. 
These  may  run  in  any  direction  in  the  veins.  In  some 
mines  the  richer  portions  take  the  form  of  patches  of 
limited  area  called  POCKETS.  A  very  rich  pocket  in 
a  gold  mine  is  usually  called  a  GLORY  HOLE.  Often 
ore  chutes  cannot  be  distinguished  by  appearance  from 
the  rest  of  the  vein  and  their  location  and  limits  can 
be  determined  by  assay  only.  There  may  be  one  such 
chute  or  a  number  of  them  in  a  mine. 

When  the  segregations  of  rich  ore  take  an  elong- 
ated form,  thick  at  the  center  and  tapering  toward 
each  end,  they  are  called  LENSES. 

[4] 


Model  of  an  Arizona  Gold  Mine  5 

This  mine  is  on  a  large  ore  chute.  After  the  vein 
had  been  found,  the  prospector  explored  it  along  its 
length  until  he  found  a  place  where  it  was  unusually 
rich.  This  was  the  outcrop  of  a  chute.  As  the  mine 
grows  deeper  the  bottom  of  the  shaft,  while  still  in 
the  vein,  will  leave  the  ore  body.  The  working  faces 
of  this  mine  at  these  deeper  levels  will  be  off  to  the 
left  and  will  be  connected  to  the  shaft  by  a  long 
gallery. 

In  the  upper  part  of  the  vein,  called  the  OXID- 
IZED ZONE  on  account  of  the  nature  of  the  minerals 
contained  in  it,  the  distribution  of  the  gold  is  much 
more  irregular  than  it  is  in  the  SULPHIDE  ZONE 
below.  In  the  oxidized  zone  the  ore  in  the  main  ore 
chute  is  again  segregated  into  minor  chutes.  Three 
of  these  are  plainly  shown  by  the  worked-out  areas 
pitching  do^\^lward  and  to  the  left  which  appear  in  the 
upper  part  of  the  model. 

Entrance  to  the  mine  is  through  the  SHAFT 
which  appears  at  the  left.  This  is  a  vertical  shaft. 
To  suit  other  conditions,  shafts  in  many  mines  are 
inclined.  Such  shafts  are  sometimes  called  INCLINES. 
In  still  other  mines  it  has  been  found  better  to  run  a 
nearly  horizontal  gallery  into  a  hillside.  This,  which 
is  practically  a  shaft  laid  on  its  side,  is  a  TUNNEL. 
The  shaft  in  the  model  is  divided  into  two  COMPART- 
MENTS and  is  hence  a  two-compartment  shaft.  To 
the  right  is  the  HOISTING  COMPARTMENT  through 
which  the  ore  is  raised  and  through  which  entrance 
and  exit  are  ordinarily  made.  The  smaller  compart- 
ment to  the  left  is  the  LADDER-WAY.  In  this  are 
placed  the  various  pipes  and  electric  wires  which  enter 
the  mines,  as  well  as  ladders  for  emergency  use.  In  this 
mine  the  ladders  are  continuous.  This  is  unusual.  In 
most  mines  the  ladder-way  is  interrupted  every  thirty 
feet  or  so  by  a  platform  to  minimize  the  danger  of  falls. 

[5] 


6  Field  Miisettm  of  Natural  History 

In  PROSPECTS,  that  is,  preliminary  workings 
to  determine  the  value  and  nature  of  an  ore  deposit, 
the  shafts  usually  have  but  one  compartment  and  in 
some  districts  of  small  mines  one  compartment  shafts 
are  common.  On  the  other  hand  larger  mines  usually 
have  more  than  two  compartments.  Three  and  four 
compartment  shafts  are  usual. 

The  shaft  is  strengthened  by  a  skeleton  lining  of 
heavy  timber.  Horizontal  rectangular  frames  spaced 
at  frequent  intervals  are  firmly  held  in  place  against 
the  rock  by  wooden  wedges.  These  frames,  called 
SETS,  are  connected  as  a  continuous  framework  by 
upright  posts  at  the  four  corners.  Other  uprights 
mark  the  division  between  the  compartments.  Cross 
timbers  in  each  set  complete  the  timbering  and  sepa- 
rate the  compartments. 

Near  the  surface,  where  the  shaft  passes  through 
soil  and  broken  rock,  further  protection  is  necessary. 
This  is  provided  by  LAGGING,  which  is  a  plank  lining 
to  the  shaft. 

In  the  hoisting  compartment  is  the  CAGE.  This 
is  an  elevator  much  like  the  elevator  of  an  office  build- 
ing. On  this  cage,  the  cars  of  ore  are  raised  and 
access  to  the  workings  is  provided  for  supplies  and 
men.  Over  the  top  of  the  shaft  is  the  HEAD-FRAME. 
This  carries  at  its  summit  a  pulley  over  which  the 
cable  from  the  cage  passes  to  the  HOISTING  EN- 
GINE housed  in  a  building  at  the  rear.  At  the  bottom 
of  the  model  the  shaft  is  continued  to  form  a  kind  of 
cistern  called  the  SUMP  in  which  the  mine  waters 
collect.  On  the  lower  level  of  the  mine  to  the  left  of 
the  shaft  is  the  PUMP  driven  by  an  electric  motor. 
This  takes  the  water  from  the  sump  and  delivers  it  to 
the  COLUMN,  a  large  pipe  in  the  ladder-way,  through 
which  it  passes  to  the  surface. 

[6] 


Model  of  an  Arizona  Gold  Mine  7 

THE  LEVELS.  When  the  shaft  had  been  sunk 
fifty  feet  a  small  chamber  called  a  STATION  was 
excavated  at  the  right.  From  this  station  a  long 
gallery  called  a  DRIFT  was  excavated  along  the  length 
of  the  vein.  A  drift  differs  from  other  passage  ways 
in  mines  in  that  it  always  runs  lengthways  of  the 
deposit. 

This  drift  and  the  workings  connected  with  it 
constitute  the  FIFTY  FOOT  LEVEL,  so  called  be- 
cause its  drift  is  fifty  feet  below  the  surface. 

When  the  shaft  had  been  sunk  a  hundred  feet  at  a 
point  near  the  base  of  the  model,  another  station  was 
cut  and  another  drift  started.  This  drift  and  its 
workings  constitute  the  HUNDRED  FOOT  LEVEL. 

On  the  hundred  foot  level  to  the  right  of  the  shaft 
is  a  gallery  running  directly  into  the  model.  This  is  a 
CROSS-CUT,  so  called  because  it  cuts  across  the  de- 
posit. This  cross-cut  runs  through  the  vein  and  into 
the  country  rock  behind.  It  was  driven  to  find  out  if 
there  was  another  vein  parallel  with  the  one  worked. 

From  the  lower  level  two  shaft-like  openings 
extend  downward.  From  the  upper  level  a  similar 
opening  extends  down  to  the  left.  These  are  WINZES 
which  were  driven  where  some  features  of  the  vein 
suggested  that  exploration  in  this  direction  might  be 
of  value. 

Near  the  right  of  the  model  a  vertical,  shaft-like 
opening  extends  from  the  lower  level  to  the  surface. 
This  is  a  RAISE,  so  called  because  it  was  excavated 
from  below  upwards.  The  reason  for  driving  it  from 
below  is  that  in  mining  it  is  much  cheaper  to  work 
from  below  than  from  above.  This  raise  is  lined  with 
CRIBBING,  that  is,  logs  built  up  log-house  fashion. 
The  raise  has  two  objects.  By  providing  a  second 
opening  to  the  surface  it  permits  a  circulation  of  air 
and  furnishes  much  needed  ventilation.     It  is  also  the 

[7] 


8  Field  Museum  of  Natural  History 

final  stage  of  blocking  out  the  ore  that  lies  between  it 
and  the  shaft.  It  will  be  seen  that  the  block  of  ore 
lying  between  the  hundred  and  the  fifty  foot  levels 
and  between  the  shaft  and  the  raise  has  four  of  its 
six  faces  open  for  examination.  Experience  has  sho'w^l 
that  when  four  faces  of  a  body  of  ore  can  be  examined, 
a  very  fair  estimate  may  be  made  of  the  quantity  and 
value  of  the  ore.  Such  ore  is  said  to  be  BLOCKED 
OUT  or  ORE  IN  SIGHT.  On  this  model  two  such 
blocks  are  shown,  one  from  the  surface  to  the  fifty 
foot  level  and  the  other  from  the  fifty  foot  to  the 
hundred  foot  level.  The  mine  workings  described, 
shafts,  drifts,  cross-cuts,  raises,  etc.,  constitute  DE- 
VELOPMENT WORK  which  must  be  done  before  the 
extraction  of  the  ore  in  a  large  way  can  be  undertaken. 
It  is  customary  to  carry  development  considerably 
ahead  of  the  mining  proper  so  that  it  may  be  known 
what  may  be  expected  of  the  mine  for  some  time  in 
the  future. 

Some  ore  is  extracted  during  development  but  at 
a  cost  much  greater  than  that  of  the  average  cost  of 
mining. 

The  larger  part  of  the  ore  is  taken  from  chambers 
called  STORES. 

The  principle  on  which  mining  is  carried  out  in 
this  mine  and  in  the  great  majority  of  all  metal  mines, 
is  to  so  conduct  the  operations  that  the  ore  always 
moves  downward  to  the  shaft  and  no  lifting  of  the  ore 
is  performed  except  at  the  shaft.  Ore  moves  down- 
ward by  its  own  weight,  but  to  raise  it,  effort,  human 
or  mechanical,  must  be  employed.  It  is  economical 
to  concentrate  all  this  application  of  power  in  one 
operation  at  the  shaft.  Stopes  worked  along  these 
lines  are  called  OVERHAND  STOPES. 

The  beginning  of  a  stope  is  shown  about  midway 
of  the  lower  level,   where   a  small   opening  appears 

t8] 


Model  of  an  Arizona  Gold  Mine  9 

above  the  drift.  Here  a  narrow  niche  has  been  exca- 
vated to  the  side  of  the  drift  and  a  raise  has  been 
driven  a  few  feet  into  the  ore  of  the  roof.  A  miner 
may  be  seen  blasting  out  the  beginning  of  a  stope  above 
this  raise.  Between  this  and  the  shaft  appear  two 
stopes  in  operation.  These  two  have  in  the  progress 
of  the  work  run  together,  making  one  large  stope. 
There  are  many  ways  of  conducting  mining  operations 
in  a  stope  and  the  methods  employed  must  be  adapted 
to  the  conditions  in  the  mine.  The  method  employed 
here  is  called  shrinkage  stoping  and  the  stope  is  a 
SHRINKAGE  STOPE.  The  ore  is  blasted  from  the 
roof  and  allowed  to  lie  on  the  floor  of  the  stope.  As 
the  broken  ore  occupies  more  space  than  it  did  when 
unbroken,  if  all  were  allowed  to  remain  it  would  more 
than  fill  the  stope.  It  is  therefore  drawn  off  from 
time  to  time  from  below  but  leaving  enough  behind  so 
that  miners  standing  on  it  may  reach  the  roof  con- 
veniently. Standing  on  this  broken  ore  they  drill 
holes  in  the  ore  above  them,  using  for  the  purpose  a 
special  type  of  air  drill,  called  a  stoper.  The  com- 
pressed air  for  these  drills  comes  from  an  air  com- 
pressor on  the  surface  in  a  small  building  behind  the 
shaft.  It  is  led  down  the  shaft  and  along  the  drifts 
in  metal  pipes.  From  these  pipes  it  is  conducted  to 
the  drills  in  the  stopes  by  rubber  hose.  When  the 
proper  number  of  holes  are  drilled  they  are  loaded 
with  dynamite  and  after  the  men  have  left,  the  holes 
are  fired,  breaking  more  ore.  In  a  small  stope  on  the 
lower  level,  the  holes  have  been  loaded  and  a  miner  is 
preparing  to  light  the  fuses.  The  excess  ore  is  then 
drawn  from  below  and  the  operation  repeated.  Access 
to  the  stope  is  by  a  manway  with  ladders  which  is  kept 
open  through  the  broken  ore  by  timbering.  At  the 
bottom  of  the  stope  near  the  entrance  of  the  manway 
is  a  chute  equipped  with  a  gate  through  which  the  ore 
is  drawn  off  into  ore  cars  in  the  drift  below.     These 

[9] 


10  Field  Museum  of  Natural  History 

ore  cars  run  on  a  tramway  to  the  shaft.  At  the  shaft 
tliey  are  run  on  to  the  cage  and  taken  to  the  surface. 
The  broken  ore  in  the  stope  not  only  provides  a  work- 
ing platform  for  the  miners  but  also  serves  in  place  of 
timbers  to  keep  the  sides  of  the  opening  apart.  At  the 
upper  left  corner  of  one  of  the  stopes  a  small  raise  has 
been  driven  to  the  level  above  for  ventilation. 

When  all  the  ore  has  been  broken,  the  continued 
withdrawal  of  the  broken  ore  from  below  would  leave 
the  stope  empty.  Left  empty  it  would  in  time  cave 
and  the  motion  of  the  rock  thus  produced  might  dis- 
turb other  workings  of  the  mine.  To  avoid  this  after 
the  stope  has  been  completed,  as  fast  as  ore  is  drawn 
from  below,  waste  rock  is  brought  from  the  surface 
to  the  fifty  foot  level  and  fed  into  the  stope  through 
this  ventilation-raise.  The  stope  will  thus  remain  full 
even  after  all  ore  has  been  withdrawn.  On  the  upper 
level  there  are  a  number  of  worked  out  stopes  which 
have  been  filled  from  the  surface.  Not  all  stopes  are 
thus  filled  however,  since  in  many  cases  conditions  are 
such  that  caving  will  do  no  harm,  and  there  are  some- 
times other  reasons  for  leaving  them  empty.  In  some 
systems  of  mining  caving  is  even  invited  and  is  so 
controlled  as  to  be  of  great  assistance  to  the  mining 
operations. 

On  the  lower  level  and  against  the  raise  are  two 
stopes  where  on  account  of  the  value  of  the  ore,  none 
has  been  left  to  form  a  floor.  Here  the  timbering  of 
the  drift  has  been  made  heavier  and  a  floor  of  poles 
laid  over  it  to  hold  the  broken  ore. 

In  one  of  the  open  stopes  of  the  upper  level  the 
walls  incline  to  bulge  inwards  so  that  broken  ore  left 
in  the  stope  would  bind  and  could  not  be  drawn  ofl". 
In  this  stope  ore  has  been  removed  as  fast  as  broken. 
The  walls  are  kept  apart  by  round  cross  timbers  called 

[10] 


Model  of  an  Arizona  Gold  Mine  11 

STULLS.     The  miners  work   on  temporary  wooden 
platforms  laid  on  the  stulls. 

The  drifts  are  made  more  secure  by  TIMBERING. 
As  the  rock  is  firm  and  as  these  levels  are  so  near  the 
surface  that  there  is  little  pressure,  not  many  timbers 
are  necessary.  For  a  long-  way  from  the  shaft  the 
drifts  have  no  timbers  whatever.  Towards  the  right, 
where  the  rock  is  more  broken,  sets  of  timber  designed 
to  hold  the  roof  of  the  upper  level  consist  of  two 
POSTS,  one  on  each  side  supporting  a  CAP. 

The  corresponding  place  on  the  lower  level  is 
provided  with  full  SETS  of  timber  consisting  of  two 
POSTS,  a  CAP  and  a  SILL. 

Near  the  right  of  the  model  is  a  vein  of  another 
kind  which  runs  at  right  angles  to  the  face  of  the 
model.  This  vein  carries  silver  and  lead.  In  the 
upper  part  the  ore  is  an  impure  carbonate  of  lead 
stained  with  iron  oxides.  In  the  lower  part,  it  is 
the  dark-colored  sulphide  of  lead,  galena.  The  silver 
content  of  the  ore  is  disseminated  in  invisible  form 
through  the  lead-bearing  minerals.  This  vein  DIPS 
to  the  left.  The  left  wall  overhangs  the  vein  and  is 
the  HANGING  WALL.  The  wall  to  the  right  is  the 
FOOT  WALL.  This  vein  and  its  country  rock  are  so 
broken  that  the  method  of  mining  employed  on  the 
gold-bearing  vein  cannot  be  used  and  exploitation 
must  be  by  more  costly  methods. 

On  the  upper  level  the  stope  runs  upward  from 
the  drift  and  will  eventually  break  through  at  the 
surface.  The  stope  is  left  empty  and  is  kept  open  by 
numerous  cross  timbers  (STULLS).  At  the  lower 
level  the  tendency  to  cave  is  too  great  to  be  resisted 
by  stulls  and  an  elaborate  system  of  timbering  called 
SQUARE  SET  is  employed.  This,  which  is  often  em- 
ployed in  deep  mines  and  where  the  ground  has  much 

[11] 


12  Field  Museum  of  Natural  History 

tendency  to  move,  consists  of  a  continuous  system  of 
hollow  squares  framed  from  heavy  timbers.  In  this 
mine  these  squares  are  ten  feet  on  edge  but  under 
worse  conditions,  they  have  to  be  made  much  shorter. 

Henry  W.  Nichols. 


This  model  is  exhibited  on  the  second  floor  of  the  Museum  in 
an  alcove  in  Frederick  J.  V.  Skiff  Hall. 


[12] 


